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Gustav’s Effect on this Course Fall Holiday has been cancelled, which means our class will meet on Thursday, 9 October. (This makes up for one class day lost to Gustav last week.) We will hold an additional makeup class on Saturday, 20 September! (This will account for the second class day lost to Gustav last week.) Date of Exam #1 has been changed to Tuesday, 23 September!

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Astronomers’ Magnitude System Ancient, Greek astronomers made catalogues of all the (visible) stars in the sky –Name –Position on the sky (angular coordinates) –Any observed motion? –Brightness on the sky (hereafter, apparent brightness)  The Greeks defined a “magnitude” system to quantify the (apparent) brightness of each star.

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Astronomers’ Magnitude System The brightest stars were labeled “1 st magnitude” stars Successively fainter stars were catalogued as 2 nd magnitude, 3 rd magnitude, etc. Faintest stars (visible to the “naked eye”) were catalogued by Greek astronomers as 6 th magnitude stars. Astronomers continue to use this “magnitude” system, extending it to much fainter objects (that are visible through telescopes but were not bright enough to be seen by Greek astronomers). The Sun can also be put on this “magnitude” system.

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Apparent brightness due to… Each star’s intrinsic brightness Each star’s distance from us A star of a given intrinsic brightness will appear to get fainter and fainter if you move it farther and farther away from us

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Concept of Apparent Brightness 10 stars that are identical in every respect (all having, for example, the same intrinsic brightness) will appear to have different brightness in the night sky if they are all at different distances from us. Apparent brightness varies as the “inverse square” of the distance. –Move a star twice as far away, it becomes 4 times fainter –Move a star 3 times farther away, it becomes 9 times fainter –Move a star 10 times farther away, it becomes 100 times fainter –Move a star to half its original distance, it becomes 4 times brighter –Move a star to 1/10 th its original distance, it becomes 100 times brighter

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Wien’s Law for Blackbody Spectra As the textbook points out (§5-4), there is a mathematical equation that shows precisely how the wavelength (color) of maximum intensity varies with gas temperature.